Osteoporosis is a major health threat for more than 28 million Americans. In the U.S., eight million women and two million men already have osteoporosis. More than 18 million Americans have low bone mass placing them at increased risk for osteoporosis.

Here are some warning signs that your bones are in trouble making you susceptible to osteoporosis later in life:

• You break a bone. Breaking a bone as an adult does not always mean you have osteoporosis-but it could be a warning sign that your bones are weak.

• You have a family history of osteoporosis or hip fractures. If someone in your family has osteoporosis, you have a 60 to 80 percent chance of getting the condition, too. And if your mother has broken a hip, your chances double for having a hip fracture-a strong indicator of bone weakness.

• You do not get enough calcium. People need to incorporate calcium throughout their lives to prevent osteoporosis. Milk and other dairy products can provide a major boost of bone-building calcium to most diets. Leafy green vegetables are another good source of calcium.

• You smoke or drink excessive amounts of alcohol. If you smoke, you double your risk of suffering an osteoporotic fracture. Researchers have found that smoking reduces your ability to absorb calcium. That's because cigarette smoke is thought to damage bone cells and prevent new bone growth. People who drink too much alcohol on a regular basis are prone to bone loss and fractures due to poor nutrition as well as increased risk of falling.

• You are thinner than normal. Low body weight is an important risk factor for osteoporosis and increased risk of fractures. Being too thin means you have less padding of muscle and fat to protect your bones which are more vulnerable to injury. Frequent dieting can be risky because when you lose weight, you also lose fat, muscle and bone density. Fat and muscle may eventually come back, but bone could be gone forever.

• You have chronic medical problems. If you suffer from rheumatoid arthritis, hyperthyroidism (excessive production of thyroid hormones), hyperparathyroidism (a condition caused by excessive amounts of parathyroid hormone), diabetes or liver disease, your chances of having osteoporosis increase.

• You have persistent back pain. Back pain that will not quit could be a sign that you have a spinal fracture. Spinal fractures occur when bones in your back become so weak that they fracture and collapse. This can happen without warning even during ordinary activities such as bending over to pick up a newspaper.

• You are shorter than you used to be. If you fracture a bone in your spine, it could collapse onto itself causing you to shrink. Multiple fractures also can cause the spine to form a curve causing the disfigurement known as a dowager's hump. However, not all height loss is caused by osteoporosis. As you age, the disks in your back lose their elasticity causing your frame to shift downward. It is not uncommon to lose anywhere from 1/2 - to 1-inch in height between the ages 60 and 80.

For more information on "Prevent Injuries America!^®," call the American Academy of Orthopaedic Surgeons' public service telephone number 1-800-824-BONES (2663).